A hydraulic working oil, which is a power transmission fluid used for such actuation as power transmission, force control and force cushioning in a hydraulic system including hydraulic machinery and equipment, also fulfils the function of lubricating sliding parts.
With the recent progress towards miniaturization and high output of modern hydraulic machinery and equipment, a working pressure has increasingly been made higher, for example, the previous pressure of 14 to 20 MPa has been increased to the present pressure of 30 MPa or higher, while an oil tank has been directed to a small capacity. Such being the case, the heat load imposed upon a working oil has been made severer than ever before, thus bringing about several problems such as premature deterioration of a working oil, sludge formation therein, the phenomenon of cylinder waviness and malfunction of hydraulic machinery or equipment.
As a hydraulic working oil, there has heretofore been used a zinc alkyldithiophosphate (ZnDTP) which is imparted with both antioxidative and lubricating capabilities. However in the case of such a hydraulic working oil, the above-mentioned ZnDTP is thermally decomposed at locally hot portions that are caused by the heat of bubble compression accompanying the trend towards high pressure. The aforesaid decomposition results in several problems such as sludge formation, the resultant malfunction of hydraulic machinery and equipment and/or the generation of an abnormal odor.
Under such circumstances, an investigation has been made on the simultaneous use of the ZnDTP and a metallic detergent. Nevertheless, such combination is not satisfactory with regard to the service life thereof, and the problem of the phenomenon of cylinder waviness at the time of operation due to high pressure still remains unsolved.